In the near future, optical free space communications between satellites and satellites and ground stations will represent an important supplement to existing microwave technology, including weight-savings on board the satellites. So-called optical terminals comprise one or more telescopes, which limit the angular area of the field of vision of an optical receiver in the direction toward a counter station, and also provide the directional beaming of the signals to be transmitted. Furthermore, several movable mirrors are provided, by means of which the alignment of the transmitting and receiving directions is performed. Besides the direct detection of the optical output of the transmitter of the counter station constituting the transmission method, the coherent superimposition of the received light with the light of a local oscillator laser having the same frequency plays an important role since, besides a great sensitivity to the signal to be detected, the insensitivity to interferences by other light sources present in the background is important.
Several lasers are required in connection with all the methods mentioned hereinafter. On the one Hand, in order to provide the light output for the optical transmitter, on the other hand, to generate the light output for a so-called beacon. This is an optical transmitter which radiates in an unmodulated manner, which, compared to the transmitter intended for communications, emits a significantly increased power to a large spatial angle, in order to make it possible for a counter station to align the transmitting beam with its own receiver.
Coherent methods additionally need a local oscillator laser, on whose light the received light is superimposed in the photodetector of the receiver. Several types of laser can be selected for all these purposes. Although diode lasers, which have reached a high state of development because of their extensive application in fiber-optic communications, represent an alternative at least for simple systems operating with intensity modulation, which in addition also saves space and weight, they are generally not yet suitable for coherent transmissions, in spite of an operation on only a single optical frequency having been achieved here, too, aside from complicated structures with large, additionally coupled resonators. One reason is the still too great spectral width of this single radiated optical frequency. Although fiber-optical coherent transmission systems also operate with commercially available diode lasers, detection is performed at relatively high optical output because of the waveguide transmission.
Existing background of other light sources, which interferes with free space transmissions, as well as the mostly very low power of the received signal, however, necessitate an optical bandwidth of the unmodulated signal, which is considerably narrower than the modulation bandwidth. These are criteria which, together with small size and low weight, can be best met by diode laser-pumped solid state lasers. Existing attempts to integrate the laser systems necessary for operation in a terminal for optical free space communications have been described by Carlson et al. and Marshalek et al. (R. T. Carlson et al., "Monolithic Glass Block Lasercom Terminal: Hardware Proof of Concept and Test Results", SPIE, vol. 2381, Free space Laser Communication Technologies VII, Feb. 7-8, 1995, San Jose, Calif., pp. 90 to 102; R. G. Marshalek et al., "Lightweight, High-Data-Rate Laser Communications Terminal for Low-Earth Orbit Satellite Constellations", SPIE vol. 2381, Free space Laser Communication Technologies VII, Feb. 7-8, 1995, San Jose, Calif., pp. 72 to 82).
Both groups of authors describe laser systems which are mechanically coupled to the optical system of a terminal and conduct their light emissions via collimated beams into the optical device. However, diode lasers have been used in this example of the prior art.
Diode laser-pumped solid state lasers have a larger volume and reduced efficiency, therefore they generate a larger amount of waste heat than a diode laser. The increased amount of heat produced in the vicinity of the optical system has been shown to be a risk for the undisturbed operation of the optical system.
The insufficient modulation capacity of diode laser-pumped solid state lasers presents a further problem. In contrast to diode lasers, the medium generating the optical gain remains for a relatively long time in the excited state after pump energy was supplied. Furthermore, the resonator of such lasers is considerably larger than that of diode lasers. Accordingly, for amplitude modulation, for example, cut-off frequencies of approximately 100 kHz are the rule. The external modulation required because of this is fairly hard to provide, since a high optical output must be handled, which demands the employment of electro-optical modulators with low cut-off frequencies.
The external modulation of laser light can be provided at high cut-off frequencies in modulators in which light is conducted in a waveguide which permits a short distance between each of the electrodes that provide the modulating voltage, and therefore permits a reduced modulation voltage. Since this method only permits low optical power because of the great increase of the optical power density caused by the narrow cross section of the optical waveguide, it is necessary to boost the modulated optical signal. Attempts to do this consist on the one hand in applying methods and devices which in the meantime have proven themselves in fiber-bound optical communications, for example by boosting the modulated optical signal by means of a fiber amplifier doped with erbium (T. Araki, M. Yajima, S. Nakamori, Y. Hisada, "Laser Transmitter Systems for High-Data-Rate Optical Inter-Orbit Communications", SPIE vol. 2381, Free space Laser Communication Technologies VII, Feb. 7-8, 1995, San Jose, Calif., pp. 264 to 272).
Besides diode laser-pumped solid state lasers, appropriate traveling wave amplifiers are used, wherein devices are made available which are particularly suitable for boosting light of lasers operated with the same techniques, particularly for the diode laser-pumped neodymium-YAG solid state lasers, which are very convenient for optical free space communications because of their narrow spectral width. The light to be amplified is conducted into an amplifying crystal, in which the photons of the optical beam will encounter with a certain probability atoms which are in an optical excited metastable state, which has a comparably long temporal stability due to the special properties of the material. The relative stability of this state is now disturbed by a photon of the same energy as the difference between the excited state and a lower energy state of the atom, in the process of which the respective atom emits an additional photon with the same phase and the same energy, i.e. the same wavelength.
The excited state of the atoms is produced by the so-called pump light, which has a shorter wavelength than the light to be amplified and puts the atoms into an excited state which corresponds to the energy of the photons, from which they pass over into a relatively stable state, whose energy difference to the lower laser level corresponds to the energy of the photons of the light to be amplified. A large amplification of the light is achieved, if the photons of the light to be amplified encounter many excited atoms when passing through the amplifying medium. Accordingly, the volume density of excited atoms must be high. However, since a defined portion per unit of time of the excited atoms, because of a finite average lifetime of the excited state, spontaneously returns to the ground state, and the photon emitted in this process is lost for the amplification of the light, to reach a high volume density of excited atoms, it is necessary to continuously radiate pump light at a high rate into the medium, even if light to be amplified is lacking, in order to obtain the high volume density of excited atoms, because of which the efficiency of such devices is extremely poor at high amplification factors. Very low amplification factors can be observed if the light to be amplified already has a high intensity, i.e. if a large average rate of photons passes through the amplifying medium and the density of excited atoms is low because of a high rate of stimulated emissions of additional photons.
Each atom excited by the pump light photon is placed into the ground state after an, on average, short time by a photon of the light to be amplified. With a comparably long average lifetime of the excited atoms, there is a comparatively low probability of a spontaneous and therefore useless change into the basic state, so that with low amplification the efficiency is high.
In order to achieve high amplification and, at the same time, a high rate of stimulated transitions into the ground state, it is necessary, in spite of the low density of excited atoms in the amplifying medium, to assure a high average number of additional photons generated by stimulated transitions of excited atoms into the ground state. In most cases this is achieved in that the light to be amplified is conducted on as many paths as possible through the zone of an amplifying medium which is irradiated by pump light. By means of this, with a respectively constant volume density of excited atoms, there is a multiple, corresponding to the number of passages, of the probability of a single passage generating additional photons for each coupled-in photon of the light to be amplified.
In spite of a low pump power it is therefore possible to achieve an considerable amplification factor. However, the devices in accordance with the prior art are constructed of several elements requiring a large amount of space and mass, which therefore only poorly meet space travel-specific requirements. Special designs also include the risk of insufficient mechanical ruggedness (T. J. Kane, E. A. P. Cheng, B. Nguyen, "Diode-Pumped ND:YAG Amplifier with 52 dB Gain", SPIE vol. 2381, Free space Laser Communication Technologies VII, Feb. 7-8, 1995, San Jose, Calif., pp. 273 to 284; T. E. Olson, T. J. Kane, W. M. Grossmann, H. Plaessmann, "Multipass Diode-Pumped ND:YAG Optical Amplifiers at 1.06 mm and 1.32 mm", Optics Letters, vol. 6, No. 5, May 1994, pp. 605 to 608).
An additional problem for space travel applications lies in that the diode lasers, which are also employed for generating the pump light, have a limited lifetime. It is accordingly necessary to keep a plurality of redundant diode lasers in readiness for each diode laser-pumped solid state laser and each diode laser-pumped optical amplifier in order to be able to replace failures.